1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission.
2. Description of the Prior Art
An automatic transmission has a transmission mechanism that is comprised of rotational elements such as planetary gear sets and engagement elements such as clutches and brakes that engage and disengage the rotational elements. The engagement elements are respectively actuated by oil pressure, and a plurality of gear positions are obtained by engaging and disengaging the engagement elements in predetermined combinations.
FIG. 13 shows a hydraulic circuit of an oil pressure that is supplied to the engagement elements.
The hydraulic circuit generating an engagement element pressure Pc that is supplied to an engagement element 60 is constituted by a regulating valve 62 for regulating a line pressure generated by a not-shown oil pump by means of pressure reduction control and outputting as an output pressure Po and a switching valve 61 for switching the output pressure Po generated by the regulating valve 62 to the line pressure and outputting as the engagement element pressure Pc to the engagement element 60.
The regulating valve 62 uses a solenoid pressure (SOL pressure) and the output pressure Po as operation signal pressures and regulates the line pressure from zero pressure to a maximum pressure.
In the switching valve 61, one end of a spool is biased by a spring 63, and the output pressure Po is applied to the other end of the spool. If a pressing force generated by the output pressure Po from the regulating valve 62 exceeds a pressing force generated by the spring 63, the switching valve 61 is a valve that switches to take a position to supply the line pressure to the engagement element 60.
Therefore, as seen from oil pressure characteristics of the engagement element pressure Pc shown in FIG. 14, a gain is controlled by a small proportional characteristics till the engagement element pressure Pc reaches a set switching pressure Pco, and if the engagement element pressure Pc becomes higher than the set switching pressure Pco, the gain instantly rises to reach the line pressure by a switching operation of the switching valve 61.
In short, an accurate oil pressure control can be performed by proportional characteristics of a small gain in the shift transition period, and the engagement element pressure Pc is changed to the line pressure after shift termination, whereby torque capacity required for keeping engagement can be ensured (for example, Japanese Laid-Open Patent Publication No. 2001-12588).
If oil pressure is supplied at the time of requirement of drain of oil pressure or is drained at the time of requirement of supply of oil pressure due to a failure of a hydraulic control device, clutches or brakes are engaged or disengaged inversely with control of the hydraulic control device to be brought into an interlocked state or a neutral state. However, an oil pressure switch 65 for detecting oil pressure condition of the engagement element pressure Pc is provided to a supply oil path for supplying the engagement element pressure Pc to the engagement element 60, whereby supply or drain of the oil pressure is detected so that a control to avoid an improper supply or drain of the oil pressure can be performed.
However, in this conventional hydraulic control device for the automatic transmission, once the engagement element is engaged, line pressure is supplied directly to the engagement element. Oil pressure including hydraulic vibration acts on the engagement element, and an effective value of the oil pressure including vibration actually determines the torque transmission capacity of the clutch. Therefore, if hydraulic vibration occurring due to the structure of the oil pump is included in the line pressure, the hydraulic vibration is directly applied to the oil pressure switch.
A mean value of the oil pressure including vibration is called an effective pressure. In the case of high-frequency hydraulic vibration, even if the effective pressure of the oil pressure applied to the engagement element is low, a momentary high pressure exceeding an allowable pressure which is allowed as an input oil pressure to the oil pressure switch acts on that oil pressure switch due to action of the hydraulic vibration. Since the oil pressure switch and peripheral hydraulic circuits thereof are configured to have a high responsiveness, the direct application of the hydraulic vibration exceeding the allowable pressure to the oil pressure switch causes a failure thereof.